what is null

null is the value of a reference variable when it doesn't point to anything in memory. Think of it as a street sign with the name of a city on it, but no arrow, and the city doesn't exist. ;)

You can compare against it in if-sentences to see if objects have been instantiated yet. Objects are null up until the point someone instantiates them using the "new" keyword:

 object test; // null
 
 test = new object(); // No longer null, points to an actual object

Read more here:

http://en.wikipedia.org/wiki/Null_pointer#Null_pointer